A gas turbine engine generally includes a fan and a core arranged in flow communication with one another with the core disposed downstream of the fan in the direction of the flow through the gas turbine. The core of the gas turbine engine generally includes, in serial flow order, a compressor section, a combustion section, a turbine section, and an exhaust section. With multi-shaft gas turbine engines, the compressor section can include a high pressure compressor (HP compressor) disposed downstream of a low pressure compressor (LP compressor), and the turbine section can similarly include a low pressure turbine (LP turbine) disposed downstream of a high pressure turbine (HP turbine). With such a configuration, the HP compressor is coupled with the HP turbine via a high pressure shaft (HP shaft), and the LP compressor is coupled with the LP turbine via a low pressure shaft (LP shaft).
In operation, at least a portion of air over the fan is provided to an inlet of the core. Such portion of the air is progressively compressed by the LP compressor and then by the HP compressor until the compressed air reaches the combustion section. Fuel is mixed with the compressed air and burned within the combustion section to provide combustion gases. The combustion gases are routed from the combustion section through the HP turbine and then through the LP turbine. The flow of combustion gasses through the turbine section drives the HP turbine and the LP turbine, each of which in turn drives a respective one of the HP compressor and the LP compressor via the HP shaft and the LP shaft. The combustion gases are then routed through the exhaust section, e.g., to atmosphere.
To maximize fuel burn in a high performance turbine engine, it is desirable to have a fan with a high bypass ratio combined with a small core. To reduce engine cost, engine weight and engine complexity, it is desirable to use differential bearings to support the high pressure shaft from the counter-rotating low pressure shaft. However, because the lengths of the cores of modern engines are tending to become longer and longer and the counter-rotating shafts are tending to rotate faster and faster, this type of configuration results in unacceptably high loads on the differential bearing that otherwise might desirably be used to support the high pressure shaft from the counter-rotating low pressure shaft. In order to support these otherwise unacceptably high loads requires the addition of frame structure, which adds weight, cost and complexity to the overall design, and thus is undesirable.
Various dynamic issues invariably will arise during the extended operation of the differential bearing. Accordingly, the ability of the differential bearing to tolerate and mitigate these dynamic issues can improve its capacity, life and reliability and thereby lower the frequency of the engine maintenance. Additionally, providing proper lubrication and cooling to the differential bearing that supports the counter-rotating shafts is necessary to maximize the life of the differential bearing and the load capacity of the differential bearing. Thus, any improvement in the tolerance of the differential bearing to deal with anticipated dynamic issues must not adversely affect proper lubrication and cooling to the differential bearing.